"Tina and Margot got really close when they worked together on Whiskey Tango Foxtrot a few years ago, but things aren't the same since Robbie's career skyrocketed," an insider told RadarOnline.com.
Long before hitting it big with popular turns as Harley Quinn and Tonya Harding, Robbie played war correspondent Tanya Vanderpole in Fey's war dramedy, Whisky Tango Foxtrot.
When the movie came out, Robbie and Fey declared themselves BFFs, with Robbie calling the 30 Rock creator and star "so chill."
But since the movie's 2016 release, Robbie has been busy.
"It's not like they've had a huge falling out," the insider added. "But Tina can go cold on people if she feels threatened, and she's pulled back from the friendship and given Margot her space."
Now friends of the two former besties are hopeful for a reunion.
"It's sad because they got along so well and their mutual friends still can't understand why they've let the friendship die in this way," the insider confessed. "They're urging them to start making time for each other again."
The source added, "They're both workaholics who could use some more fun in their lives!"
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